1. Field of the Invention
The present invention relates to a styrenic resin composition. More particularly, the present invention relates to a styrenic resin composition comprising a rubber modified styrene maleic anhydride (SMA) copolymer and polybutene; to articles of manufacture, e.g. thermoformed containers suitable for packaged foods that are to be heated in microwave ovens, that are produced from the styrenic resin composition and having improved properties, e.g. toughness, elongation, and heat distortion resistance; and to related methods for producing the styrenic resin composition.
2. Background Art
It is known to copolymerize styrene and maleic anhydride. Such processes have been described at length in the literature, especially in Baer U.S. Pat. No. 2,971,939 and Hanson U.S. Pat. No. 2,769,804, and beneficially as a solution as disclosed in U.S. Pat. No. 3,336,267.
It is known in the art to modify styrene maleic anhydride (SMA) copolymers with rubber. Generally, these copolymers are referred to as “rubber modified styrene/maleic anhydride copolymers”. It is known that the rubber component provides increased impact resistance and that the maleic anhydride component provides a high heat distortion temperature. An improved method for preparing styrene/maleic anhydride/diene rubber composition suitable for extrusion and molding and having a high heat distortion temperature and desired impact resistance is disclosed in Moore et al. U.S. Pat. No. 3,191,354 (The Dow Chemical Company), which was issued on Nov. 11, 1975.
Hathaway et al. U.S. Pat. No. 5,219,628 (The Dow Chemical Company), which was issued on Jun. 15, 1993, discloses a multi-layer container for use in the microwave cooking of food. The container comprises a substrate layer of thermoplastic polymer that is not suitable for contact with the food, and an inner layer comprised of a blend of styrene/maleic anhydride copolymer and a polymer selected from the group consisting of polystyrene, rubber modified polystyrene, polymethyl methacrylate, rubber modified polymethyl methacrylate, polypropylene, and mixtures thereof. This patent also teaches that rubber modified styrene/maleic anhydride copolymers may also be used, but are not preferred.
It is known to produce various shaped articles from foamed and unfoamed thermoplastic materials such as polystyrene sheet or impact modified polystyrene sheet (i.e. high impact polystyrene sheet) by thermoforming methods. Many such articles are containers used for packaged foods.
Chundury et al. U.S. Pat. No. 5,106,696 (assigned to Ferro Corporation), which was issued on Apr. 21, 1992 discloses and claims a thermoformable multi-layer structure for packaging materials and foods. A polymer composition for a first layer of the structure comprises: (A) 49% to 90% by weight of a polyolefin, i.e. polypropylene, polybutene; (B) 10% to 30% by weight of a copolymer of styrene and maleic anhydride; (C) 2% to 20% by weight of a compatilizing agent, i.e., a starblock, diblock or mixtures thereof of a copolymer of styrene and butadiene; (D) 0 to 5% by weight of a triblock copolymer of styrene and butadiene; and (E) 20% by weight of talc. The second layer of the structure is made of polypropylene.
It is known to improve the environmental stress crack resistance (ESCR) of high impact polystyrene (HIPS) and other impact modified styrenic polymers, such as acrylonitrile-butadiene-styrene plastic (ABS) and methyl methacrylate-butadiene-styrene plastics (MBS), with the addition of polybutene. U.S. Pat. No. 5,543,461 assigned to Novacor Chemicals (International) S.A. discloses a rubber modified graft thermoplastic composition comprising: 1) 99 to 96% by weight of a rubber modified thermoplastic comprising: (a) 4 to 15 weight % rubbery substrate, preferably polybutadiene, that is distributed throughout a matrix of the superstrate polymer in particles having a number average particle size from 6 to 12 microns and (b) 96 to 85% by weight of a superstrate polymer; and 2) 1 to 4% by weight of polybutene having a number average molecular weight from 900 to 2000. Claim 10 of this patent recites that the superstrate polymer may comprise 85% to 95% by weight of styrene and from 5% to 15% by weight of maleic anhydride. The ESCR of the impact modified styrenic polymers is attributed to the large particle size of the impact modifier, i.e. 6 to 12 microns and to the use of the low molecular weight polybutene. Such thermoplastics find a fairly significant market in housewares, which are subject to chemicals that tend to cause environmental stress cracking (ESC), such as cleaners and in some cases, fatty or oily food.
U.S. Pat. No. 5,543,461 discussed in the preceding paragraph discloses in the background section that the thermoplastic having the best ESCR is Chevron's HIPS grade 6755. This Chevron product contains 2 to 3 weight % of polybutene and has a dispersed rubbery phase with a volume average particle diameter between 4 and 4.5 microns. This Chevron product relates to high impact polystyrene (HIPS) with ESCR properties and not to a rubber modified styrene/maleic anhydride copolymer.
A number of process designs are disclosed in the patent literature involving polymerization techniques, reactor configurations and mixing schemes that are used to incorporate maleic anhydride in a styrene/maleic anhydride copolymer. Examples include Tanaka et al. U.S. Pat. No. 4,328,327 assigned to Daicel Chemical Industries, Ltd., Meyer et al. U.S. Pat. No. 4,921,906 assigned to Stamicarbon B. V., and the above Moore et al. U.S. Pat. No. 3,919,354 assigned to The Dow Chemical Company.
The latter document, i.e. U.S. Pat. No. 3,919,354 discloses an improved styrene/maleic anhydride/diene rubber composition suitable for extrusion and molding and having a high heat distortion temperature and desired impact resistance. The process for the preparation of the polymer involves modifying a styrene-maleic anhydride copolymer with diene rubber by polymerizing the styrene monomer and the anhydride in the presence of the rubber. More particularly, the process involves providing a styrene having rubber dissolved therein; agitating the styrene/rubber mixture and initiating free radical polymerization thereof; adding to the agitated mixture the maleic anhydride at a rate substantially less than the rate of polymerization of the styrene monomer; and polymerizing the styrene monomer and the maleic anhydride. The polymer contains rubber particles ranging from 0.02 to 30 microns dispersed throughout a matrix of polymer of the styrene monomer and the anhydride with at least a major portion of the rubber particles containing occlusions of the polymerized styrene monomer and maleic anhydride. This patent teaches that the polymers are suited for extrusion into sheet or film, which are then employed for thermoforming into containers, packages and the like. Alternately the polymers can be injection molded into a wide variety of components such as dinnerware and heatable frozen food containers.
However, polymers as those disclosed in the above U.S. Pat. No. 3,919,354 are generally brittle, and therefore, capable of breaking even though these polymers have the thermal properties to withstand temperatures above 210° F., which temperature is generally used in heating food in a microwave oven.
It is desirable to have an article such as a container that is suitable for packaged foods and that could withstand the temperatures needed for heating foods in a microwave oven without the container breaking, especially upon removal of the container out of the microwave oven.